MEMS based over-the-air optical data transmission system

ABSTRACT

Building-to-building over the air transmission of optical data is a growing area of data communications. The fast growing use of bandwidth mandates the use of over the air transmission equipment capable of similar performance as the performance of the fiber optic transmission, for distances of 3-10 Km. Transparent transmission is important, to enable seamless growth from low data-rare to Gbps rates, and then to Dense Wavelength Division Multiplexed (DWDM) transmission of several wavelengths. The only way to achieve the required performance is with narrow, directable beams. This patent application discloses Micro-Electro-Mechanical-Systems (MEMS) mirror based, over the air, optical data transmission system. A narrow optical beam is used and a MEMS mirror fine-tunes the aiming of the beam to track building movement, vibrations etc.

[0001] This non-provisional application takes priority from U.S.Provisional Application No. 60/210,613 filed on Jun. 9, 2001.

BACKGROUND OF THE INVENTION

[0002] A description of some technologies related to embodiments of theinvention follows:

[0003] U.S. Pat. No. 4,662,004 Fredriksen, et al. Fredriksen describesoptical communication link that includes a separate laser (in additionto the data transmission laser), which returns information about thelevel of the received signal to the transmitter. This separate laser isadjusted to emit power proportional to the received beam power.

[0004] U.S. Pat. No. 4,832,402 Brooks. Brooks describes a fast scanningmirror used to time-multiplex light beam into several steering mirrors,each of the steering mirrors aim the beam into one or a group of targetsclustered together. The steering mirrors are slow due to the large anglerequired. Brooks also describes the use of “beacon transmitters” to aidin target tracking (column 9 line 15).

[0005] U.S. Pat. No. 5,282,073 Defour, et al. Defour shows opticalcommunications system with two galvanometer mirrors for beam steering,and a complex wide-angle lens to increase the angular scanning to ahalf-sphere. Defour also describes target designation step iterativestep of bilateral acquisition and a third step of exchanging data.

[0006] U.S. Pat. No. 5,390,040 Mayeux. Mayeux describes the use of onesteer-able mirror at the expanded beam location, for aiming both thetransmit beam and receive beam. Part of the surface of the mirror isused for transmission, and another part for reception. (Mayeux callsthese parts of the mirror “field of views”, in contrast to commonterminology).

[0007] U.S. Pat. No. 5,448,391 Iriama, et al. Iriama describes the useof optical Position Detector sensor (common art) to track the beamdirection. A pair of mirrors is used for slow, large angle directioncontrol and a fast lens is moved for fast corrections.

[0008] U.S. Pat. No. 5,646,761 Medved, et al. Medved describes here anoptical communications between stationary location like an airport gateand a movable object, like an airplane parked at the gate. The opticalunits on the gate and the airplane are searching for each other and stopthis search when aligned.

[0009] U.S. Pat. No. 5,710,652 Bloom, et al. Bloom describes opticaltransmission equipment to interconnect low Earth orbit satellites. Thewhole transmitter and receiver unit is mounted on gimbals. Two lasersare used, one for tracking and one for data. A CCD optical detectordetects the target location for tracking servo control.

[0010] U.S. Pat. No. 5,768,923 Doucet, et al. Doucet discloses thedistribution of Television signals from one source to many receivers.The transmitter uses X-Y beam deflector made of two galvanometer drivenmirrors. This assembly is used to direct the beam into a specificreceiver at a selected home.

[0011] U.S. Pat. No. 5,818,619 Medved, et al. Medved describes here acommunications network with air-links. A converter unit is convertingthe physical data transmission in the network to electricity, and drivesan air-link transmitter. Similarly, the received beam is converted toelectricity after reception. Medved also describes an optical switch tohave one air-link serving plurality of networks between the same twolocations.

[0012] EP 962796A2 Application Laor, et al. This application describesMEMS mirror construction.

SUMMARY OF THE INVENTION

[0013] Optical interconnect with light beams between buildings suffersfrom a difficulty associated with the movement of the buildings. Themovements include waving in the wind, environmental vibrations, landshift, earthquakes etc. Common over-the-air optical transmissionequipment either uses narrow beam laser transmitters with trackingmechanisms or use LED based wide beam transmitters with fixed aiming.

[0014] MEMS is a technology that is used to manufacture small mechanicalsystems using common Silicon foundry processes. We describe here the useof narrow field of view transmission with MEMS mirror being used to finetune the beam direction. Since the MEMS mirror is rather small, 1-3millimeters in diameter, it is difficult, if not impossible to use it toaim the expanded beam. In an embodiment of the invention, the MEMSmirror is installed near the light source, where the beam is small indiameter. This positioning enables only small angular deflection of thebeam. The transmission equipment will be aimed coarsely manually or withmotors, and the MEMS mirror will do fine aiming with fast response. Withcourse motorized aiming, the motors may be operated to search and findthe other side of the communication link. After the MEMS mirror begunaiming the beam, the motors could be adjusted slowly to hold the aimsuch that the MEMS mirror average angular deviation is around zero. Thiswill maximize the correction capability of the MEMS mirror.

[0015] Note: we will use here “light” for all electromagnetic waves fromthe ultra-violate to infrared, and not only for the visible spectrum.This is a common use of the term. The common transmission wavelength iswith light in the near infrared, and not only for the visible spectrum.This is a common use of the term. The common transmission wavelength iswith light in the near infrared between 600 and 1600 nano-meters.

[0016] Another feature of the invention is the use of optical fiber tocarry light from the light source in the date equipment to the opticalbeam transmitter on the roof or in a window. Another optical fibercarries the light from the optical beam receiver on the roof or in awindow to the detector in the data equipment. This facilitates thechanging of data equipment, changing data rates, changing protocols,etc. without the need to replace the optical beam transmitter or beamreceiver. The system may be upgraded to carry light in more then onewavelength using the same optical beam transmitter and receiver. Forlong transmission lengths, an optical fiber amplifier could be installedbetween the light source and the optical beam transmitter, or betweenthe optical beam receiver and the detector, or both locations. Forsystems located in areas with common fog problems, such amplifiers couldbe set to kick-in when transmission is fading.

[0017] Yet another feature is the use of two fast optical fiber 1×Nswitches to time-share the use of a network between several users. Onenetwork port will connect to the switches, with two fibers—transmit andreceive. On the other side of the switches each pair of fibers will beconnected to a pair of an optical transmitter and an optical receiver,aimed at one network user. This enable to begin serving high data ratenetwork interconnect to customers in a time-shared fashion, and adjustthe percentage of time used according to the needs of each customer.When the need arises, a dedicated network port could be used todirect-connect a customer for a full connection. The structure of thesystem having fully transparent optical transmitters and receivers allowfor seamless transfer to the use of dedicated fibers between the twolocations when such fibers are installed.

[0018] A construction is described where the beam transmitter and thebeam receiver share the use of one MEMS mirror. Servo control of theMEMS mirror angular position may be achieved with separate servo LEDsource and servo optical position detector. Close loop servo control iscritical to the correct operation of the transmission system.

DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows the construction of a beam transmitter or beamreceiver unit in accordance with one embodiment of the invention.

[0020]FIG. 2 is a schematic drawing showing the movement of the image ofthe optical fiber end in accordance with one embodiment of theinvention.

[0021]FIG. 3 is the MEMS mirror drawn showing the mirror and package inaccordance with one embodiment of the invention.

[0022]FIG. 4 shows a different optical design of the beam transmitter inaccordance with one embodiment of the invention.

[0023]FIG. 5 is an example of a mechanism for course aiming inaccordance with one embodiment of the invention.

[0024]FIG. 6 is a different azimuth-elevation structure in accordancewith one embodiment of the invention.

[0025]FIG. 7 shows a network system using the beam transmitters andreceivers described in accordance with one embodiment of the invention.

[0026]FIG. 8 shows fiber amplifiers being inserted into a communicationlink in accordance with one embodiment of the invention.

[0027]FIG. 9 shows a system where several sub networks are served by onemain network in accordance with one embodiment of the invention.

[0028]FIG. 10 shows the possible use of one MEMS mirror to control boththe transmitted beam and the received beam in accordance with oneembodiment of the invention.

[0029]FIG. 11 shows the design of a MEMS mirror serving bothtransmission and reception in accordance with one embodiment of theinvention.

[0030]FIG. 12 shows a servo LED being used as a light source inaccordance with one embodiment of the invention.

[0031]FIG. 13 shows the servo sensor, which uses the same MEMS mirror asdescribed in accordance with one embodiment of the invention.

[0032]FIG. 14 shows an outside view of the beam transmitter and receiverunit in accordance with one embodiment of the invention.

[0033]FIG. 15 shows a flattened drawing of the optical system of FIG.14.

DETAILED DESCRIPTION

[0034] The invention comprises a method and apparatus for MEMS basedover-the-air optical data transmission system. In the followingdescription, numerous specific details are set forth to provide a morethorough description of embodiments of the invention. It will beapparent, however, to one skilled in the art, that the invention may bepracticed without these specific details. In other instances, well knownfeatures have not been described in detail so as not to obscure theinvention.

[0035]FIG. 1 shows the construction of a beam transmitter or beamreceiver unit in accordance with one embodiment of the invention. In abeam transmitter, the light that propagates in the optical fiber isexiting the fiber end in a cone. The optical fiber is a common SingleMode telecommunications fiber, with core diameter of approximately 10microns and cladding diameter of 125 microns. The cone of light hits theMEMS mirror and is deflected towards the lens, which collimates the beamfor transmission. The collimation may not be exact, as larger or smallerbeam angles may be required. The mirror may be rotated in two degrees offreedom over two perpendicular axis (not shown) which are parallel tothe mirror surface. The image of the optical fiber end is thus moved inspace. By moving the image of the optical fiber, the beam that emergesfrom the lens change direction.

[0036]FIG. 2 is a schematic drawing showing the movement of the image ofthe optical fiber end in accordance with one embodiment of theinvention. Light cone emerges from the fiber core at the fiber end. Thiscone is reflected by the MEMS mirror. The mirror is rotate-able aroundthe axis shown, and the second axis is not shown for clarity. When themirror is in position A, the mirror creates an image A and the lightexits in cone A. When the mirror is in position B, the mirror creates animage B and the light exits in cone B. Since image A and B are indifferent positions, the lens will collimate light exiting from theseimages in different directions. The two exiting cones have some beamwander on the lens, requiring somewhat larger lens diameter.

[0037] In FIG. 3, the MEMS mirror is drawn showing only the mirror andpackage. The package is a mechanical structure that holds and protectsthe MEMS mirror. The mirror package may have a window that enableshermetic sealing, not shown here for clarity. The MEMS mirror can becontrolled to rotate in the horizontal and vertical axis. A detaileddescription of the type of MEMS mirror useful for this application maybe found in “Optical Switch Demos in Cross-Connect” by David Krozier andAlan Richards, Electronic Engineering Times, May 13, 1999, p. 80 and inEP 962796A2. The MEMS mirror dimensions are reported to be approximately3 mm×4 mm. The size is larger than a typical MEMS mirror and is quiteuseful for the construction of the beam transmitter unit. A smaller MEMSmirror will require the fiber to be very near to the mirror, maybeobstructing part of the beam. Also, a small mirror will create onlysmall deviation of the position of the image of the fiber, and achievesmall active angle of aiming. The reader should not, however, that thesize of the MEMS mirror may vary in accordance with differentembodiments of the invention.

[0038]FIG. 4 shows a different optical design of the beam transmitter.The beam emerging from the fiber is collimated by an “on-axis” lens. Thecollimated beam is reflected by the MEMS mirror into an “eyepiece” lens.The eyepiece lens focuses the beam into a real image spot at or near thefocal plane of the lens. The lens creates a collimated or nearlycollimated beam for transmission. By rotating the MEMS mirror, thelocation of the real image can be adjusted, thereby adjusting thedirection of the transmitted beam.

[0039] It is common knowledge that for any path taken by a beam oflight, the reverse is also a possible path for another beam. Therefore,FIGS. 1-4 which were described above as beam transmitters could be usedto explain similar design beam receivers. A light beam arrives at thelens and being focused and directed to the fiber end by the MEMS mirror.The direction from where the fiber will accept light is controlled bythe MEMS mirror. The fiber in the beam receiver could be identical tothe fiber in the beam transmitter, but it may also be a common MultiMode fiber, with core diameter of 50 or 62.5 microns and clad diameterof 125 microns. Larger core diameter will allow relaxed aiming accuracy,but will limit the data rate if the fiber is long, due to modaldispersion.

[0040] A pair of units, a beam transmitter and a beam receiver, togethercreates an optical link. The distance between beam transmitter and beamreceiver could be several kilometers. For two-way communications, lightcan be made to propagate in the fibers in both directionssimultaneously. Alternatively, two pairs of units can be used to createa full duplex link.

[0041] The beam steering by the MEMS mirror is limited in angle. Only afew degrees of angular deviation are typically possible. In somedesigns, only a fraction of a degree of adjustment is possible.Therefore, a mechanism for course aiming is required, that is capable ofaiming in 360 degrees in azimuth and approximately +/−45 degrees inelevation. FIG. 5 is an example of such mechanism. The beam transmitter(or receiver) is mounted onto a mount, with a motor that controls thehorizontal axis of rotation of the beam transmitter/receiver. This motorenables the movement of the beam in elevation. The exact design of themotor and movement mechanism are not shown since it is a common art. Themount is attached to the base with similar drive, which enables rotationaround the vertical axis, for adjusting the beam direction in azimuth.The motors are capable of aiming the beam generally to the target, butare neither fast nor accurate enough to track the building movements.

[0042]FIG. 6 is a different azimuth—elevation structure. The beamtransmitter or receiver is mounted on a base facing up. A large foldingmirror directs the beam in a general horizontal direction. The beamtransmitter (receiver) and the folding mirror rotate around verticalaxis for azimuth control. It is possible that only the folding mirrorwill rotate to achieve azimuth control. The mirror aims the beam inelevation by rotating around a horizontal axis. Again, the motor driveis not shown since it is common art.

[0043]FIG. 7 shows a network system using the beam transmitters andreceivers described above. The main network needs to interconnect withthe sub network. The main network and the sub network are located indifferent buildings with free line-of-sight between them. Also possibleis interconnect between different floors of the same building by sendingthe beams vertically. A network element is attached to the main network,such as a switch, router and the like. A port in the network element isconnected to the beam transmitter and receiver with a pair of fibers. Alaser or LED transmitter and a PIN or avalanche photodiode detector atthe network element performs the light generation and detectionrespectively, commonly marked TX and RX. The beam transmitter andreceiver are mounted on the roof or in a window, aimed at the beamtransmitter and receiver which are connected to the sub network withfibers. When the beam units are correctly aimed at each other, lightfrom the TX unit at each network element is passing via the fiber to thebeam transmitter, over the air to the beam receiver and to the RX unitat the other network element. A full duplex communication isestablished.

[0044] Since the network elements sees standard fibers attachments, itis very simple to connect direct point-to-point optical fibers whenavailable, replacing the over-the-air link. This feature allows forseamless growth of the network.

[0045] Opitcal transmission from the TX unit to the RX unit will sufferlosses, due to loss in the fibers, optical aberrations and diffractionin the beam transmitter and receiver, the receiver aperture beingsmaller in diameter then the beam generated by the beam transmitter,inaccuracies in the aiming servo mechanism for both transmitter andreceiver, optical absorption and scattering in the atmosphere, etc. Incommon 2.5 Gbps transmission equipment such loss is allowed to reach20-30 dB, i.e. only {fraction (1/100)} to {fraction (1/1000)} of thelight transmitted by the laser should arrive at the detector to achievelow error rate transmission. If the link loss is excessive, fiberamplifiers could be inserted in the link as shown in FIG. 8. The opticalfiber amplifiers that are commonly used are Erbium Doped FiberAmplifiers (EDFA). An amplifier may be inserted into the link after thelaser to boost the transmitter power, or before the receiver to increasethe received optical power, or in both locations. If the high loss is aphenomenon related only to fog condition, the amplifiers may be insertedactively when the bit error rate deteriorates.

[0046]FIG. 9 shows a system where several sub networks are served by onemain network. 1×N fiber optics switch is attached to the TX in the mainnetwork. The switch is serving light to one of the beam transmitters ata time. A second switch is connected to the RX. Each sub networkoperates for a short time, and then is disconnected for a longer time.For example, the switching time may be 5 mS and each sub network couldbe served for 100 mS at a time. If there are 5 sub networks, there willbe a gap of 425 mS between connections for any specific sub network.Some messages may be delayed, but this may be tolerated. If the linkloss is different to different sub networks, the gain of the opticalamplifier may be adjusted to each sub network differently. Fast AGC isrequired on all the RX units. This construction enables the installationof standard transmission equipment, for example Gigabit Ethernet, in allthe network elements, even when the communications needs is lower, andadjusting the main network connect time to each sub network according tothe needs. An advantage is the use of only two optical amplifiers, whichare expensive. Another advantage is that the connectivity to each subnetwork may be adjusted without the need for a physical equipmentchange, and remotely. The user of the sub network may be charged fornetwork services according to the average data rate he uses. Only when asub network needs full connectivity at the main network data rate, thenthis sub network could be assigned a port in the main network and directconnection instead via the fiber switches.

[0047]FIG. 10 shows the possible use of one MEMS mirror to control boththe transmitted beam and the received beam. The transmit fiber is shownhaving Numerical Aperture (NA) of 0.1, which is common for Single Modefibers, and creates an opening of the beam at about 5.7 degrees from theaxis. The beam reflects from the MEMS mirror and is aimed at thetransmit lens via a fixe mirror. The receive fiber is shown having NA of0.26, which is common for Multi Mode fibers with core diameter of 62.5microns. The received beam will have radius of about 15 degrees. Sinceit is intended to use the same area of the MEMS mirror for bothtransmission and reception, the transmit and receive cones can not haveparallel axis at the MEMS mirror. The fixed lens is used, therefore, tomake the transmit and receive beams parallel outside of this combinedbeam transmitter and receiver.

[0048]FIG. 11 shows the design of a MEMS mirror serving bothtransmission and reception, where the beams at the MEMS mirror aresubstantially collimated. The description of each optical path, fortransmission and reception, is essentially the same as described forFIG. 4.

[0049] The operation of the atmospheric optical link depends criticallyon the correct aim of the transmit and receive beams. A servo controlmust be employed to aim the beams. The servo system should have adifferent mechanism t align the beams then the data beams, and manydifferent ways are known and described in the prior art. We need,however, a mechanism that makes use of the positioning of the same MEMSmirror as the transmit and receive beams. The essential parts of such aservo mechanism are shown in FIGS. 12 and 13. In FIG. 12, a servo LED isused as the light source. Laser could also be used. The servo LED emitslight modulated at relatively low speed, enabling detection with lowreceived power. The servo LED lens creates a wide cone of light from thelight emitted by the servo LED. This cone may be several degrees wide,so the aiming is very simple and the amount of detected radiation is notsensitive to small movements of this beam. FIG. 13 shows the servosensor, which uses the same MEMS mirror as described before. The sensoruses optical position detector, which is a common art and includesSilicone diode with several outputs. The electrical signals outputtedfrom the detector are sensitive to the intensity of the optical signaland to the exact location of the optical signal on the detector. Theelectrical signals indicate if the MEMS mirror is aiming the servosensor beam directly at the opposing servo LED. If there is an error inaiming, the electrical signal outputted from the detector indicate thedirection and magnitude of the error. The servo system will then adjustthe MEMS mirror correctly.

[0050]FIG. 14 shows the outside view of the beam transmitter andreceiver unit. In FIG. 15 a flattened drawing of the optical system ofFIG. 14 is shown. The optical beams are shown by the central beam only,for clarity. One MEMS mirror is used to control three beamsconcurrently.

[0051] Thus, a method and apparatus for MEMS based over-the-air opticaldata transmission system has been described. However, the claims and thefull scope of their equivalents describe the invention.

1. An atmospheric optical transmitter comprising: an optical transmitterfor transmitting an optical signal; an optical fiber for carrying saidoptical signal from a source to said optical transmitter: a MEMS mirrorfor reflecting said optical signal transmitted by said transmitter; aMEMS mirror adjuster for adjusting said MEMS mirror to aim saidreflected optical signal.